The term split-cycle engine as used in the present application may not have yet received a fixed meaning commonly known to those skilled in the engine art. Accordingly, for purposes of clarity, the following definition is offered for the term split-cycle engine as may be applied to engines disclosed in the prior art and as referred to in the present application.
A split-cycle engine as referred to herein comprises:
a crankshaft rotatable about a crankshaft axis;
a power piston slidably received within a power cylinder and operatively connected to the crankshaft such that the power piston reciprocates through a power (or expansion) stroke and an exhaust stroke during a single rotation of the crankshaft;
a compression piston slidably received within a compression cylinder and operatively connected to the crankshaft such that the compression piston reciprocates through an intake stroke and a compression stroke during a single rotation of the crankshaft; and
a gas passage interconnecting the power and compression cylinders, the gas passage including an inlet valve and an outlet (or crossover) valve defining a pressure chamber therebetween.
For purposes of clarity, the following is a list of acronyms for the various engine operating modes described herein:
AC Air compressor;
AM Air motoring;
CB Compression braking;
ICE Internal combustion engine;
PAP Pre-compressed air power;
PCA Pre-compressed combustion air.
U.S. Pat. No. 6,543,225 B2, U.S. Pat. No. 6,609,371 B2 and U.S. Pat. No. 6,952,923, all assigned to the assignee of the present invention, disclose examples of split-cycle internal combustion engines as herein defined. These patents contain an extensive list of United States and foreign patents and publications cited as background in the allowance of these patents. The term “split-cycle” has been used for these engines because they literally split the four strokes of a conventional pressure/volume Otto cycle (i.e., intake, compression, power and exhaust) over two dedicated cylinders: one cylinder dedicated to the high pressure compression stroke, and the other cylinder dedicated to the high pressure power stroke.
Considerable research has been recently devoted to air hybrid engines as compared, for example, to electric hybrid systems. The electric hybrid system requires the addition to the conventional four stroke cycle engine of batteries and an electric generator and motor. The air hybrid needs only the addition of an air pressure reservoir added to an engine incorporating the functions of a compressor and an air motor, together with the functions of a conventional engine, for providing the hybrid system benefits. These functions include storing pressurized air during braking and using the pressurized air for driving the engine during subsequent starting and acceleration.
However, the prior art appears to involve only adapting a conventional four stroke cycle engine to perform the compression, combustion and motoring functions in a single cylinder. This, then, requires a complex valve and drivetrain system and control which is capable of switching from a compression-braking (CB) mode to an air motoring (AM) mode and back to a conventional internal combustion engine (ICE) mode during operation.
In a typical example, when not storing or utilizing compressed air to drive the vehicle, a prior art air hybrid engine operates as a conventional internal combustion engine, where the four strokes of the Otto cycle (intake, compression, power and exhaust) are performed in each piston every two revolutions of the crankshaft. However, during the compression-braking mode, each cylinder of the conventional engine is configured to operate as a reciprocating piston two-stroke air compressor, driven from the vehicle wheels by vehicle motion. Air is received from outside atmosphere into the engine cylinders, compressed there, and displaced into an air-reservoir. Work performed by the engine pistons absorbs the kinetic energy of the vehicle and slows it down or restricts its motion. In this way the kinetic energy of the vehicle motion is transformed into energy of compressed air stored in the air reservoir.
During the air motoring mode, each cylinder of the engine is configured to utilize the stored compressed air to produce power strokes for propulsion without combustion. This may be accomplished by first expanding the stored, compressed air into the cylinders to drive the pistons down from top dead center (TDC) to bottom dead center (BDC) for a first power stroke. Then the pistons compress the expanded gas as they travel from BDC to TDC. Fuel is then injected into the cylinders and ignited just before TDC. The expanding products of combustion then drive the pistons down again for a second power stroke on the second revolution of the crankshaft.
Alternatively, air-motoring may be accomplished by expanding the stored compressed air to drive the power piston down from TDC to BDC for a power stroke without combustion for each revolution of the crankshaft. This alternative method of air motoring may continue until the pressure in the air reservoir falls below a threshold level, whereupon the engine may switch to either the previously described air motoring mode or a conventional ICE engine mode of operation.
Problematically, in order to switch among the CB, AM and ICE modes, the valve/drive train system becomes complex, costly and hard to control or maintain. Additionally, since each cylinder must perform all of the functions for each mode, they cannot be optimized easily. For example, the pistons and cylinders must be designed to withstand an explosive combustion event, even when just acting as an air compressor. Accordingly, due to the tolerances and materials required to withstand the heat of combustion, some sacrifice must be made to the efficiency of the compressor mode.
Another problem with performing all functions for each mode (ICE, CB and AM) in every cylinder is that no two modes can be performed in parallel (i.e. simultaneously). Because prior art air hybrid systems utilize conventional engines, they are restricted to operating in each mode serially, which imposes inherent limitations on their capabilities. For example, because the CB mode cannot be utilized when the engine is operating as an internal combustion engine (in ICE mode), the air reservoir can only be charged during the braking function of a moving vehicle. This limitation leads to problems in maintaining the stored charge in the air reservoir. Additionally, this limitation also means that prior art air hybrid systems are only applicable to moving vehicles, and are not practical for stationary applications such as stationary generators.